The caged nut idea is a good one (oddly I used to work in a hospital for caged nuts....); I had a friend who used dzus fastners on his TA, and they didn't look so good. They will work though, because the receptor is a spring clip-like arangement that can be fixed in the recess in the tank that held the rubber washer. If you haven't lost the lug itself it's pretty easy to repair the panel with that. Screwing into the back of the lug is an excellent idea, and will provide good support.
On the other hand, I have used Isopon P40 (available from any car accessory shop) to support and fix the lug, and I've never had one fail on me in two TAs! The idea is that you use the glass-fibre reinforced epoxy resin to glue and support the repair. I usually glue the lug with isocyanate superglue first, to get it into the right position, then apply the P40 around that. You will need to use a knife to deeply scratch the plastic on the panel and on the lug, and both have to be clean of course. You will need to make sure the P40 doesn't extent onto the area thatfits inside the rubber washer, or the lug will then be too fat to fit. It works and is much cheaper than either the dedicated kits that are available specifically for fairing repair, or a set of dzus fastners. If you do go down the latter route, try to get stainless dzus fastners, as the others rust.
Whichever method you choose, good luck!!
Tall men see everyone else's bald patch but their own.......